crypto/md5: provide a top-level Sum function

Makes it easy to ask the simple question, what is the hash of this data?
Also mark block as non-escaping.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/10624044

src/pkg/crypto/md5/md5.go

@@ -88,7 +88,11 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := *d0
+	hash := d.checkSum()
+	return append(in, hash[:]...)
+}
 
+func (d *digest) checkSum() [Size]byte {
 	// Padding.  Add a 1 bit and 0 bits until 56 bytes mod 64.
 	len := d.len
 	var tmp [64]byte
@@ -118,5 +122,13 @@ func (d0 *digest) Sum(in []byte) []byte {
 		digest[i*4+3] = byte(s >> 24)
 	}
 
-	return append(in, digest[:]...)
+	return digest
+}
+
+// Sum returns the MD5 checksum of the data.
+func Sum(data []byte) [Size]byte {
+	var d digest
+	d.Reset()
+	d.Write(data)
+	return d.checkSum()
 }

src/pkg/crypto/md5/md5_test.go

@@ -53,6 +53,10 @@ var golden = []md5Test{
 func TestGolden(t *testing.T) {
 	for i := 0; i < len(golden); i++ {
 		g := golden[i]
+		s := fmt.Sprintf("%x", Sum([]byte(g.in)))
+		if s != g.out {
+			t.Fatalf("Sum function: md5(%s) = %s want %s", g.in, s, g.out)
+		}
 		c := New()
 		buf := make([]byte, len(g.in)+4)
 		for j := 0; j < 3+4; j++ {

src/pkg/crypto/md5/md5block_decl.go

@@ -6,4 +6,6 @@
 
 package md5
 
+//go:noescape
+
 func block(dig *digest, p []byte)